Transparent conductive films comprise electrically conductive material coated on high-transmittance surfaces or substrates, and are widely used in flat panel displays such as liquid crystal displays (LCD), touch panels or sensors, electroluminescent devices (e.g., light emitting diodes), thin film photovoltaic cells, or as anti-static layers and electromagnetic wave shielding layers.
Currently, vacuum deposited metal oxides, such as indium tin oxide (ITO), are the industry standard materials for providing optical transparency and electrical conductivity to dielectric surfaces such as glass and polymeric films. However, metal oxide films are fragile and prone to damage during bending or other physical stresses. They also require elevated deposition temperatures and/or high annealing temperatures to achieve high conductivity levels. For certain substrates that are prone to adsorbing moisture, such as plastic and organic substrates (e.g., polycarbonates), it becomes problematic for a metal oxide film to adhere properly. Applications of metal oxide films on flexible substrates are therefore severely limited. In addition, vacuum deposition is a costly process and requires specialized equipment. Moreover, the process of vacuum deposition is not conducive to forming patterns and circuits. This typically results in the need for expensive patterning processes such as photolithography.
In recent years there is a trend to replace current industry standard transparent conductive ITO films in flat panel displays with a composite material of metal nanostructures (e.g., silver nanowires) embedded in an insulating matrix. Typically, a transparent conductive film is formed by first coating on a substrate an ink composition including silver nanowires and a binder. The binder provides the insulating matrix. Thereafter, a transparent UV or thermally curable polymer material can be coated to form a protection layer. Nanostructure-based coating technologies are particularly suited for printed electronics. Using a solution-based format, printed electronic technology makes it possible to produce robust electronics on large-area, flexible substrates.
The presence of particulate nanostructures in transparent conductive films may give rise to certain optical challenges that are not typically encountered in the ITO films, which are continuous. FIG. 1 shows an ITO touch sensor (10) and a nanowire-based touch sensor (12) in a side-by-side view, both placed on top of an LCD module (14). When the LCD module (14) is turned off, the ITO touch sensor (10) appears black in the ambient light; whereas the touch sensor made from silver nanowire-based transparent films (12) has a “milkier” or “cloudier” look. Thus, there is a need to address the optical challenges unique to nanostructure-based transparent conductors.